V8: Remove extra V8::Context allocated for expressing strong references

Moved the Referencer code into QV8Engine and re-used the available v8
context there. That also makes things a bit cleaner in the sense that now
references from one object to another are guaranteed to be within the same
context. Previously some strong references would be across contexts that
do not actually share a security token.

Change-Id: I717b27a4d96323feb570023d4d84f2b2176d1a84
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>
Reviewed-by: Chris Adams <christopher.adams@nokia.com>